Flexible catheters are inserted into the body in order to measure fluid pressure in regions of interest. Catheters are inserted into the coronary arteries and into the lower urinary tract for this purpose, and are useful in other procedures also. At the distal end or somewhere along the length of the catheter, or at both places, it is known to incorporate pressure-responsive electronic sensors. These require that conductive leads extend along the catheter structure. In the course of measurement, electrical currents are introduced through them into regions of the body where stray electrical fields are considered by many to be undesirable. These devices are relatively expensive, and must be individually calibrated.
Manometer techniques have also been used for such measurements. These have the considerable advantage that they do not require the introduction of electrical currents into the body and accordingly are potentially safer. Also they are potentially much less expensive, because they are little more than small tubes which carry a liquid column whose height is proportional to the pressure being measured.
However, fluid column techniques do have problems of their own. One is that a major portion of their length will often be flexed in various and varying directions during a procedure. A bend in a flexible tube will result in a reduction of the contained volume in the bent region, which will cause the displacement of some of the liquid which it holds. This displaced fluid will be read as a change in pressure because the meniscus will move, so that there is a latent inaccuracy in measurements when flexible tubing is used in a manometer system.
Yet another important variable in measurements made with known manometric techniques is the "station elevation" (atmospheric pressure) of the community where the measurement is done. Atmospheric pressure in Denver, Colo., will always be importantly different from the atmospheric pressure at sea level. This can lead to inconsistencies in readings of the same instrument from locale to locale.
It is an object of this invention to provide a fluid-coupled cathether capable of providing a fluid pressure indication outside of the body which is inherently compensated for atmospheric pressure differences and variations, and for configuration variations such as are caused by changes in the volume of the flexible tubing used for the catheter by its being bent and unbent during use, such as may occur when a recumbent patient sits up or a standing patient bends over.